1. Field of the Invention
This invention relates to preamplifiers for use with musical instruments, and more particularly to such preamplifiers having circuitry for simulating a vacuum tube amplifier including the distortion produced by such amplifiers.
2. Background Art
Musical instrument amplifiers, traditionally incorporating vacuum tube devices, have been used with various instruments for quite some time. Electric guitars, for example, which generate essentially a "clean" or undistorted sinewave, were among the first instruments to rely heavily on vacuum tube amplification. Such amplification was typically faithful to the sound generated by the guitar, which was thus faithfully reproduced on an associated speaker.
Because vacuum tubes are inherently forgiving devices, operable in almost any circuit design, various inexpensive and amateruish design techniques began to be used with the vacuum tube amplifiers. The use of poor designs, marginal components such as cheap transformers, and the like led to a proliferation of amplifiers providing distortion caused by poor gain balancing and restricted ranges of linear operation. As a result, premature clipping or overdriving of various stages within the amplifier took place. The transformers similarly provided their own distortion to the output signal, due to saturation, for example.
The distorted output of such vacuum tube amplifiers, however, became appreciated and desired, particularly for rock 'n roll music of the later 1960's.
Thus it was that the guitar became, effectively, an inexpensive synthesizer wherein the sine waves generated by the guitar were transformed into square waves by amplifier clipping. The amplifier's overload, equalization, damping factor and loudspeaker response characteristics ultimately determined the output sound. Such output sound differed from the conventional clean guitar heard in early rock'n roll, country, surfing music, and the like. Rather, a reedy quality was produced, resembling, for example, the reedy sound of a woodwind instrument such as a clarinet in response to insertion of a single sustained note into the amplifier.
In the past one to two decades, the above described output sound has become identified with a "good" electrical guitar sound, and several attempts have been made to reproduce this sound with transistorized circuits.
Moog U.S. Pat. No. 4,180,707, for example, discloses a "fuzz box" for producing four types of clipping. Only two clipping levels are provided, however, defined therein as "soft" and "hard" clipping. Further, with no attempt at changing the frequency characteristic of the amplifier, the disclosed circuit cannot truly simulate a vacuum tube amplifier.
Jahns U.S. Pat. No. 3,973,461 discloses a vacuum tube circuit for providing varying degrees of distortion, and an essentially identical solid state circuit for replacing the tube. The circuit fails to provide compensation for increased gain by variation of frequency response, and generally does not combine gain, frequency response variation and distortion.
Laub U.S. Pat. No. 3,825,409 discloses an amplifier for use with electric guitars for introducing crossover distortion proportional to the amplitude of the input signal, otherwise suffering from the same defects of the prior art.